Splicing refers to the process by which intronic sequence is removed from the nascent pre-messenger RNA (pre-mRNA) and the exons are joined together to form the mRNA. Splice sites are junctions between exons and introns, and are defined by different consensus sequences at the 5′ and 3′ ends of the intron (i.e., the splice donor and splice acceptor sites, respectively). Alternative pre-mRNA splicing, or alternative splicing, is a widespread process occurring in most human genes containing multiple exons. It is carried out by a large multi-component structure called the spliceosome, which is a collection of small nuclear ribonucleoproteins (snRNPs) and a diverse array of auxiliary proteins. By recognizing various cis regulatory sequences, the spliceosome defines exon/intron boundaries, removes intronic sequences, and splices together the exons into a final translatable message (i.e., the mRNA). In the case of alternative splicing, certain exons can be included or excluded to vary the final coding message thereby changing the resulting expressed protein.
Regulation of the expression of a target gene (e.g., a therapeutic transgene) is necessary in a variety of situations. In the context of the therapeutic expression of genes, techniques that enable regulated expression of transgenes have the potential to enhance safety by regulating the level of expression and its timing. A regulated system to control protein expression has practical and, in some cases, essential roles for safe and effective therapeutic applications.